Process for producing multilayer printed circuit boards

ABSTRACT

A process for producing a multilayer printed circuit board produced characterized by using a special thermosetting epoxy resin composition comprising (a) an epoxy resin, (b) a crosslinking agent and (c) a polyfunctional epoxy resin, for forming a thermosetting copper-clad adhesive resin sheet, which cover an interlayer circuit plate, followed by etching of cured adhesive resin for forming holes for interstitial via holes (IVH) having a small diameter using a special etching solution comprising (A) an amine as a solvent, (B) an alkali metal compound, and (C) an alcohol as a solvent, is suitable for mass production of multilayer printed circuit boards having IVH excellent in connection reliability and electrical properties.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing a multilayer printedcircuit board.

Multilayer printed circuit boards are produced by laminating a pluralityof inner layer plates obtained by forming circuits on copper-cladlaminates and a plurality of single-sided copper-clad laminates orcopper foils via prepregs obtained by impregnating glass cloth as a basematerial with a resin, followed by pressing with heating to cure withheating, and forming circuits on outer surfaces of the integratedcopper-clad laminates containing inner layer circuits therein.

With the recent tendency towards miniaturization, higher performance,and increased functions of electronic devices, multilayer printedcircuit boards have become higher in density, thinner in individuallayers, finer in wiring, smaller in the diameter for connectingindividual layers, and have come to use interstitial via holes(hereinafter referred to as "IVHs") for connecting only neighboringwiring layers. Now, in order to make wiring density higher, a smallerdiameter of IVH is required.

A prior art multilayer printed circuit board having IVHs is produced bylaminating an inner layer circuit substrate 1 obtained by formingcircuit on a copper-clad laminate and a pair of single-sided copper-cladlaminates or copper foils 3 via a plurality of prepregs 9 as shown inFIG. 2A, adhering under pressure, with heating, to give an integratedcopper-clad laminate having interlayer circuits therein as shown in FIG.2B, drilling holes on predetermined positions so as to reach theinterlayer circuits to form holes 5 for IVH as shown in FIG. 2C, ifnecessary, drilling a through hole 6 according to a prior art process asshown in FIG. 2D, connecting the interlayer circuits and outer layercopper foils by electroless copper plating and electric copper plating 7as shown in FIG. 2E, forming etching resists 8 on the outer layer copperfoils as shown in FIG. 2F, conducting selective etchings as shown inFIGS. 2G and 2H, and removing the etching resists as shown in FIG. 2I.

According to prior art processes, polyimide films are used as a materialcapable of chemical etching using hydrazine, etc. as disclosed, forexample, in JP-A 50-4577, JP-A 51-27464, JP-A 53-49068, etc. Further,processes for etching epoxy resin cured articles used for printedcircuit boards using concentrated sulfuric acid, chromic acid,permanganic acid, etc. (surface roughness, smear treatment) aredisclosed, for example, in JP-A 54-144968 and JP-A 62-104197.

According to the prior art processes for forming holes for IVH, sinceholes are drilled until interlayer circuits are reached, it isimpossible to conduct the drilling by laminating a plurality of printedcircuit boards in contrast to drilling of through holes. Thus, thedrilling is conducted one after another, requiring a long period of timeand rendering poor productivity. Further, in order to control the depthof the point of the drill the point of the drill coincides with, thedepth of the copper wiring patterns. Since there is a variation inthickness of multilayer printed circuit boards, in some cases theinterlayer circuits are reached and in some cases they are not. In thecase of a thin intralayer thickness, holes contact with a wiring circuitof the layer thereunder, result in a defective electrical connection. Inaddition, when holes of 0.3 mm or less in diameter are drilled, the lifeof drill is remarkably shortened due to loss of core center and fromworking of resin layers containing a glass cloth substrate.

On the other hand, according to prior art chemical etching methods, theuse of hydrazine is not preferred due to its toxicity, and the use ofconcentrated sulfuric acid, chromic acid or permanganic acid is notpreferable due to designation of special chemical substances. Thesechemical substances should be avoided from the viewpoint of safety.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forproducing a multilayer printed circuit board containing interstitial viaholes (VHs) that are excellent in mass productivity, connectionreliability and electrical properties, are capable of being made thinand show excellent safety during production.

The present invention provides a process for producing a multilayerprinted circuit board, which comprises the step of:

laminating on an interlayer plate having conductor circuits thereon, athermosetting copper-clad adhesive resin sheet having a copper foil onone side, so as to make the resin side of the sheet contact with theinterlayer plate, followed by pressing with heating to cure the resinand to give an integrated laminate, said thermosetting copper-cladadhesive resin sheet having been prepared by integrating a copper foilwith a thermosetting epoxy resin composition comprising (a) an epoxypolymer having a film-forming ability and a weight-average molecularweight of 100,000 or more, obtained by polymerizing a bifunctional epoxyresin and a bifunctional halogenated phenol in an equivalent weightratio of epoxy group:phenolic hydroxyl group of 1:0.9 to 1:1.1, (b) acrosslinking agent, and (c) a polyfunctional epoxy resin, and curing theresin composition to become a B-stage condition,

forming an etching resist on the copper foil of the thermosettingcopper-clad adhesive resin sheet, followed by formation of fine holes inthe copper foil surface by selective etching,

removing the etching resist,

removing the cured resin layer under the fine holes by etching using anetching solution comprising (A) an amide as a solvent, (B) an alkalimetal compound, and (C) an alcohol as a solvent to form via holes andexpose parts of the conductor circuits,

plating a metal layer or coating an electroconductive paste so as toelectrically connect the conductor circuits of the interlayer plate andan outer layer copper foil,

forming an etching resist on the outer layer copper foil, followed byformation of wiring circuits on the copper foil by selective etching,and

removing the etching resist.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1M are schematic cross-sectional views explaining the stepsof producing the multilayer printed circuit board according to oneexample of the present invention.

FIGS. 2A to 2I are schematic cross-sectional views explaining the stepsof producing a multilayer printed circuit board according to prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, studies of various resincompositions containing no glass cloth as a base material, capable ofbeing thinned and suitable for forming holes for IVHs by chemicaletching in one simple processing have been made. As a result, inaccordance with the invention, a thermosetting epoxy resin compositionhas been found which can be subjected to chemical etching after thermalcuring, and an etching solution therefor has also been discovered.

According to the present invention, a multilayer printed circuit boardcan be produced by

laminating on an interlayer plate having conductor circuits thereon athermosetting copper-clad adhesive resin sheets, each sheet having acopper foil on one side, so as to make the resin side of the sheetcontact with the interlayer plate, followed by pressing with heating tocure the resin and to an integrated laminate, said thermosettingcopper-clad adhesive resin sheet having been prepared by integrating acopper foil with a thermosetting epoxy resin composition comprising (a)an epoxy polymer having a film-forming ability and a weight-averagemolecular weight of 100,000 or more, obtained by polymerizing abifunctional epoxy resin and a bifunctional halogenated phenol in anequivalent weight ratio of epoxy group:phenolic hydroxyl group of 1:0.9to 1:1.1, (b) a crosslinking agent, and (c) a polyfunctional epoxyresin, and curing the resin composition to become a B-stage condition,

forming an etching resist on the copper foil of the thermosettingcopper-clad adhesive resin sheet, followed by formation of fine holes inthe copper foil surface by selective etching,

removing the etching resist,

removing the cured resin layer under the fine holes by etching using anetching solution comprising (A) an amide as a solvent, (B) an alkalimetal compound, and (C) an alcohol as a solvent to form via holes andexpose parts of the conductor circuits,

plating a metal layer or coating an electroconductive paste so as toelectrically connect the conductor circuits of the interlayer plate andan outer layer copper foil,

forming an etching resist on the outer layer copper foil, followed byformation of wiring circuits on the copper foil by selective etching,and

removing the etching resist.

The thermosetting epoxy resin composition used in the present inventioncomprises (a) an epoxy polymer having a film-forming ability, (b) acrosslinking agent, and (c) a polyfunctional epoxy resin.

The epoxy polymer (a) having a film-forming ability and a weight-averagemolecular weight of 100,000 or more, preferably 100,000 to 1,000,000,can be obtained by polymerizing a bifunctional epoxy resin and abifunctional halogenated phenol in an equivalent weight ratio of epoxygroup:phenolic hydroxyl group in the range of 1:0.9 to 1:1.1, in thepresence of a catalyst, with heating, preferably in a solvent of anamide or ketone having a boiling point of 130° C. or higher, in areacted solid content of 50% by weight or less. The weight-averagemolecular weight is measured by gel permeation chromatography usingpolystyrene as a standard.

As the bifunctional epoxy resin, there can be used any compound havingtwo epoxy groups in the molecule. Examples of the bifunctional epoxyresin are bisphenol A type epoxy resins, bisphenol F type epoxy resins,bisphenol S type resins, and aliphatic hair-like epoxy resins. Thepreferred resin is bisphenol A epoxy resin. These compounds may have anymolecular weight. These compounds can be used singly or as a mixturethereof. Components other than the bifunctional epoxy resin may becontained as impurities so far as they do not affect the operation andaccomplishment of the present invention.

As the bifunctional halogenated phenol, there can be used any compoundhaving one or more halogen atoms as substituent and two phenolichydroxyl groups. Examples of the bifunctional halogenated phenol aremonocyclic bifunctional phenols such as hydroquinone, resorcinol,catechol, etc.; polycyclic bifunctional phenols such as bisphenol A,bisphenol F, naphthalene diols, bisphenols, and alkyl-substitutedhalides of these compounds. These compounds may have any molecularweight. These compounds can be used singly or as a mixture thereof.Components other than the bifunctional halogenated phenol may becontained as impurities so far as they do not affect the operation andaccomplishment of the present invention.

As the catalyst, there can be used any compounds which have catalyticactivity to accelerate the etherification reaction of epoxy group orgroups with a phenolic hydroxy group or groups. Examples of the catalystare alkali metal compounds, alkaline earth metal compounds, imidazoles,organic phosphorus compounds, secondary amines, tertiary amines,quaternary ammonium salts, etc. Among the catalysts the use of alkalimetal compounds is most preferable. Examples of the alkali metalcompounds are hydroxides, halides, organic salts, alcohlates,phenolates, hydrides, borohydrides, and amides of sodium, lithium andpotassium. These catalysts can be used singly or as a mixture thereof.

As the solvent used for the reaction, there can preferably be usedamides or ketones, and amides having a boiling point of 130° C. orhigher. Any solvents which can dissolve both the epoxy resins andphenols which are used as starting materials, can be used. Examples ofthe solvents are formamide, N-methylformamide, N,N-dimethylformamide,acetamide, N-methylacetamide, N,N-dimethylacetamide,N,N,N',N'-tetramethylurea, 2-pyrrolidone, N-methyl-2-pyrrolidone,carbamic acid esters, etc. These solvents can be used singly or as amixture thereof. As the ketones, there can be used cyclohexanone, acetylacetone, diisobutyl ketone, phorone, isophorone, methyl cyclohexanone,acetophonone, etc.

As for the polymerization conditions, the mixing ratio of thebifunctional epoxy resin and the bifunctional halogenated phenol is interms of the equivalent weight ratio of epoxy group:phenolic hydroxygroup in the range of 1:0.9 to 1:1.1.

Although the quantity of the catalyst is not particularly limited, thecatalyst, generally, is used in a quantity of 0.0001 to 0.2 mole permole of the bifunctional epoxy resin.

The polymerization reaction temperature is preferably 60° to 150° C.When the temperature is lower than 60° C., there is a tendency to makethe polymerization rate too slow. On the other hand, when thetemperature is higher than 150° C., there is a tendency to bring about aside reaction and to fail to obtain a long chain polymer.

The solid content in the reaction solution during polymerization ispreferably 50% by weight or less, more preferably 30% by weight or less.

As a result of the polymerization, the epoxy polymer (a) having amolecular weight of 100,000 or more is obtained.

As the crosslinking agent (b) for the epoxy polymer (a), there can beused masked isocyanates which are isocyanates masked (or blocked) with acompound having another active hydrogen.

The isocyanates can be any isocyanates having two or more isocyanategroups in the molecule. Examples of the masked isocyanates areisocyanate such as hexamethylene diisocyanate, diphenylmethanediisocyanate, isophorone diisocyanate, tolylene diisocyanate, etc.masked with a phenol, an oxime, an alcohol, etc. From the viewpoint ofimprovement in heat resistance of the cured article, the use ofisophorone diisocyanate or tolylene diisocyanate masked with a phenol ispreferred.

The amount of the crosslinking agent used is preferably 0.1 to 1.0equivalent weight of the isocyanate group per equivalent weight ofalcoholic hydroxyl group of the epoxy polymer (a).

As the polyfunctional epoxy resin (c), there can be used any compoundshaving two or more epoxy groups in the molecule. Examples of thepolyfunctional epoxy resin (c) are glycidyl ethers of phenols such asphenol novolac epoxy resins, cresol novolac epoxy resins, resole epoxyresins, bisphenol epoxy resins etc.; alicyclic epoxy resins, epoxidizedpoly-butadienes, glycidyl ester type epoxy resins, glycidyl-amine typeepoxy resins, isocyanurate type epoxy resins, plexible epoxy resins,etc. Among them, a phenol type epoxy resin or a mixture of a phenol typeepoxy resin and a polyfunctional epoxy resin is preferable, in order toimprove heat resistance.

The polyfunctional epoxy resin (c) can be used in an amount of 20 to100% by weight based on the weight of the epoxy polymer (a).

Further, since the polyfunctional epoxy resin (c) functions as anadhesive component and a flowing component at the time of molding, theamount used can be controlled properly, taking into account thethickness of copper foil on the interlayer, or the density of itscircuits.

The polyfunctional epoxy resin (c) can be used singly or as a mixturethereof. Further, a curing agent and a curing accelerator for thepolyfunctional epoxy resin (c) can also be used. As the curing agent andthe curing accelerator, there can be used a novolac type phenol resin,dicyandiamide, an acid anhydride, an amine, an imidazole, a phosphine,etc., singly or as a mixture thereof. Further, the addition of a silanecoupling agent is preferable for improving adhesive strength of epoxyadhesive film, particularly for improving the adhesive strength to thecopper foil.

Examples of the silane coupling agent are epoxysilane, aminosilane,ureasilane, etc.

A varnish comprising (a) the epoxy polymer, (b) the crosslinking agent,and (c) the polyfunctional epoxy resin, as mentioned above, is coated onthe copper foil and dried with heating to give a copper-clad adhesiveresin sheet of the B-stage. The preferable curing condition to get theB-stage condition is heating at 130-150° C. for 5-10 minutes. Thethickness of the resin layer is preferably 25 to 70 μm, depending on thethickness of copper foil on the interlayer circuit. The thickness ofcopper foil is preferably 5 to 70 μm.

The thermosetting copper-clad adhesive resin sheet of B-stage islaminated on one or both sides of a previously prepared interlayercircuit plate so as to face the resin side of the adhesive resin sheet,followed by pressing with heating to cure with heating. Thus, acopper-clad laminate containing interlayer circuits is obtained. Since aB-stage thermosetting resin can be cured completely at a moldingtemperature of 170° C. or more for 30 minutes or more, there can beemployed conventional molding conditions for epoxy resin laminates, solong as they satisfy the abovementioned conditions. Particularly, sincethe heating temperature and time influence the degree of curing andcause a variation in the etching speed at a later stage, it is necessaryto cure completely. Further, since the pressure influences the flow ofresins, it is necessary to employ a suitable pressure. A preferredpressure is 2 MPa or more.

Then, on one or both outer layer copper foils of the resultingcopper-clad laminate containing interlayer circuits therein bonded withthe thermosetting copper-clad adhesive resin sheet, an etching resist isformed, followed by development-selective etching by a conventionalphotography method to form fine holes on the copper foil sides. Theconventional photography method is described in Chapter 6 of "PrintedCircuits Handbook" (issued by McGraw-Hill Book Company, Editor-in-Chief:Cryde F. Coombs, Jr.). The fine holes become opening portions for IVH.Then, the etching resist is removed.

For Example, a dry film including a photosensitive resin layer (i.e.HK-425, HK-450, trade names, mfd. by Hitachi Chemical Co., Ltd.) ispreferably used as the etching resist. In this case, the dry film islaminated on the copper foil using a hot-roll laminator. The conditionsfor exposing to light, developing and removing the film, depend on thetype of the film.

The cured resin layer under the fine holes is removed by etching usingan etching solution comprising (A) an amide series as a solvent, (B) analkali metal compound, and (C) an alcohol series as a solvent so as toexpose the interlayer circuit. The epoxy polymer (a) which is acomponent of the thermosetting resin composition constituting thethermosetting copper-clad adhesive resin sheet, can be dissolved by thealkali. The etching action of the cured resin layer is conducted bycutting and decomposition of the skeleton of the epoxy resin (a) havinga high molecular weight by the alkali impregnated into the cured resinlayer accompanying the amide series solvent and the alcohol seriessolvent so as to give fragments having a lower molecular weight, whichare then dissolved in an amide series solvent to effect etching.

As the amide series solvent (A), there can be used formamide,N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide,N,N-dimethylacetamide, N,N,N',N'-tetramethylurea, 2-pyrrolidone,N-methyl-2-pyrrolidone, carbamic acid esters, etc. The use ofN,N-dimethylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidoneis preferable, since these solvents can remarkably dissolve decomposedportions of the cured product having a lower molecular weight. Thesesolvents can be used singly or as a mixture thereof. Further, thesesolvents can be used together with one or more ketone series solvents,ether series solvents and other solvents.

Examples of the ketone series solvent used together are acetone, methylethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, methyl isobutylketone, 2-heptanone, 4-heptanone, diisobutyl ketone, cyclohexanone, etc.

Examples of the other series solvent used together are dipropyl ether,diisopropyl ether, dibutyl ether, anisole, phenetole, dioxane,tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol diethylether, diethylene glycol dimethyl ether, diethylene glycol diethylether, etc.

The contents of the solvents may not be particularly limited. It ispreferable to contain the amide series solvent (A) in an amount of 50 to90% by weight based on the total weight of the etching solution, inorder to accelerate the decomposition rate and dissolving rate of thecured resin.

As the alkali metal compound (B), there can be used compounds of alkalimetals such as lithium, sodium, potassium, rubidium, cecium, etc., saidcompounds being able to be dissolved in the alcohol solvent. Examples ofthe alkali metal compounds are metals such as lithium, sodium,potassium, rubidium, cecium, etc.; hydrides, hydroxides, borohydrides,amides, fluorides, chlorides, bromides, iodides, borates, phosphates,carbonates, sulfates, nitrates, organic acid salts and phenol salts ofthe alkali metals. These alkali metal compounds can be used singly or asa mixture thereof. Among these alkali metal compounds, the use oflithium hydroxide, sodium hydroxide and potassium hydroxide isparticularly preferable from the perspective of the decomposition rateof the cured resin.

The alkali metal compound is preferably used in an amount of 0.5 to 15%by weight based on the total weight of the etching solution, in order toaccelerate the decomposition rate of the cured resin.

As the alcohol series solvents (C), there can be used methanol, ethanol,1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol, tert-butanol,1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, iso-pentylalcohol, tert-pentyl alcohol, 3-methyl-2-butanol, neopentyl alcohol,1-hexanol, 2-methyl-1-pentanol, 4-methyl-2-pentanol, 2-ethyl-1-butanol,1-heptanol, 2-heptanol, 3-heptanol, cyclohexanol, 1-methylcyclohexanol,2-methylcyclohexanol, 3-methylcyclohexanol, 4-methylcyclohexanol,ethylene glycol, ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol monopropyl ether, ethylene glycolmonobutyl ether, diethylene glycol, diethylene glycol monomethyl ether,diethylene glycol monoethyl ether, diethylene glycol monopropyl ether,diethylene glycol monobutyl ether, triethylene glycol, triethyleneglycol monomethyl ether, triethylene glycol monoethyl ether,tetraethylene glycol, polyethylene glycol, 1,2-propanediol,1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,2,3-butanediol, 1,5-pentanediol, glycerin, diproylene glycol, etc.,singly or as a mixture thereof.

Among these alcohols, the use of methanol, ethanol, diethylene glycolmonomethyl ether, diethylene glycol monobutyl ether, diethylene glycolmonoethyl ether, ethylene glycol monomethyl ether, ethylene glycolmonobutyl ether, and ethylene glycol monoethyl ether singly or as amixture thereof is particularly preferable, due to high solubility ofthe alkali metal compound.

It is preferable to use the alcohol (c) in an amount of 4.5 to 35% byweight based on the total weight of the etching solution in order toaccelerate the decomposition rate of the cured resin.

Since the contact time of the cured resin and the etching solution, andthe temperature of the etching solution, depend mutually on thedesirable etching speed and degree of etching, it is necessary to employconditions suitable for the IVH diameter and IVH thickness. Generally,the diameter is 50 μm to 1 mm. In this case, the preferable contact timeis 10 to 60 minutes and the preferable temperature of the etchingsolution is 50 to 80° C. Any etching method such as a spray method, or adip method, can be employed so long as the etching solution as mentionedabove is used. In other words, it is possible to form a fine IVH if thecombination of the thermosetting epoxy resin composition as a curedresin layer and the etching solution is used and the suitable conditionsare employed.

For the removal of decomposed material sufficiently, it is preferable towash holes with water using an ultrasonic washing device. The preferredtime for the washing is 3 to 5 minutes.

After the etching, the exposed interlayer circuits and the outer layercopper foils are electrically connected by a conventional platingmethod, such as an electric plating. When the size of IVH is small,electroless plating can be used.

Further, the electrical connection can be carried out using anelectroconductive paste, which is coated, dried and cured.

Then, an etching resist is formed on the outer layer copper foil anddevelopment/selective etching is conducted by a conventional photographymethod to form wiring circuits on the outer layer copper foil side.After removing the etching resist, there can be obtained a multilayerprinted circuit board wherein the interlayer circuits and outer layercircuits are connected by IVHs.

Using the thus produced multilayer printed circuit board as an innerlayer, laminating one or a pair of thermosetting copper-clad adhesiveresin sheets on the inner layer, and repeating this a number of times,it is possible to obtain a multilayer printed circuit board having 6 ormore layers connected by IVHs.

The present invention is illustrated by way of the following Examples,in which all parts and percents are by weight unless otherwisespecified.

EXAMPLE 1

Production of a multilayer printed circuit board is explained referringto FIGS. 1A to 1M.

A bisphenol A type epoxy resin (epoxy equivalent weight: 171.5) as abifunctional epoxy resin in an amount of 171.5 g, 271.9 g of atetrabromo-bisphenol A (hydroxyl equivalent weight: 271.9) as abifunctional halogenated phenol, and 0.78 g of lithiuum hydroxide as acatalyst were dissolved in N,N'-dimethylacetamide, as an amide seriessolvent. The solid content in the solution was 30% by weight. Thesolution was stirred by a mechanical stirrer and maintained at 120° C.for 10 hours. Finally, the solution was saturated at 5,000 mPa secviscosity and an epoxy polymer having a high molecular weight wasobtained. The weight-average molecular weight of the epoxy polymer was182,000 (gel permeation chromatography).

A resin composition comprising the epoxy polymer having a high molecularweight (a), an isophorone diisocyanate masked with a phenol resin (b),and a cresol novolac polyfunctional epoxy resin (c) (the weight ratio of(a):(b):(c)=100:16:84) was coated on a roughened copper foil (18 μmthick) to give a thermosetting copper-clad adhesive resin sheet 2 havinga resin layer (B-stage condition cured by heating at 130° C. for 10minutes) of 50 μm thick (MCF-3000E, a trade name, mfd. by HitachiChemical Co., Ltd.), which was laminated on a previously preparedinterlayer circuit plate 1 (substrate: MCL-E-67, a trade name, mfd. byHitachi Chemical Co., Ltd., shown in FIG. 1A). Then, pressing in avacuum was conducted at 170° C. under a pressure of 2 MPa for 30 minutesto give a copper-clad laminate containing interlayer circuits (FIG. 1B).

An etching resist 4 (a photosensitive dry film, H-K 450, a trade name,mfd. by Hitachi Chemical Co., Ltd.) was formed on each surface of outerlayer copper foil of the copper-clad laminate containing interlayercircuits using a hot-roll laminator (FIG. 1C). A part of the resist withdiameters of 50 to 300 μm were removed from selected positions by aphotography method, to form holes for IVH (FIG. 1D). The conditions forremoval were exposed to 50 mJ/cm² strength light and development with aNa₂ CO₃ aqueous solution. Further, from the portions in which IVHs areto be formed, a part of the outer layer copper foil was removed (FIG.1E).

Then, the etching resists were removed by NaOH aqueous solution toexpose cured epoxy adhesive film on the portions in which IVHs are to beformed (FIG. 1F).

Subsequently, an etching solution comprising 90% ofN-methyl-2-pyrrolidone, 3% of potassium hydroxide and 7% of methanolheated at 50° C. was contacted to etch the cured epoxy adhesive film for15 minutes, so as to expose the interlayer circuits (a dip method).Thus, holes for IVHs were formed. Since the removal of decomposedmaterials was insufficient, the holes were subjected to continuousultrasonic treatment by dipping the board into an ultrasonic washingdevice for 3 minutes and washing with water (FIG. 1G).

Then, a through hole 6 was drilled (FIG. 1H).

Subsequently, copper plating of 15 to 20 μm was conducted in the holesfor IVH and the through hole, so as to electrically connect theinterlayer circuits and the outer layer copper foils (FIG. 1I).

Next, an etching resist 8 (a photosensitive dry film, H-K 425, a tradename, mfd. by Hitachi Chemical Co., Ltd.) was formed on each outer layersurface using a hot-roll laminator (FIG. 1J), and subjected to selectiveetching to form wiring circuits (FIGS. 1K and 1L). After removal of theetching resists (FIG. 1M), there was obtained a four-layered multilayerprinted circuit board.

The resulting multilayer printed circuit board was subjected to ananti-corrosion test, a soldering heat resistance test and a test tomeasure the peel strength of the surface copper foil.

The anti-corrosion test was conducted by subjecting the interlayercircuit previously having a comb-like pattern with a conductor intervalof 0.1 mm to the conditions of 120° C., 85% of moisture and 100V tomeasure changes of an insulation resistance. The initial value was 10¹³Ω and changed to 10¹² Ω after 1000 hours. As to the soldering heatresistance test, no change was admitted after the solder float test at260° C. for 3 minutes. The peel strength of surface copper foil was 1.7kg/cm.

EXAMPLE 2

The process of Example 1 was repeated except for the use of an etchingsolution comprising 50% of N-methyl-2-pyrrolidone, 15% of potassiumhydroxide and 35% of methanol heated at 70° C., and conducting etchingby contacting with the cured epoxy adhesive film for 30 minutes. As aresult, a 4-layered multilayer printed circuit board having the sameproperties as in Example 1 was obtained.

EXAMPLE 3

The process of Example 1 was repeated except for the use of an etchingsolution comprising 90% of N,N-dimethylacetamide, 1% of potassiumhydroxide and 9% of diethylene glycol monomethyl ether heated at 70° C.,and conducting etching by contacting with the cured epoxy adhesive filmfor 15 minutes. As a result, a 4-layered multilayer printed circuitboard having the same properties as in Example 1 was obtained.

EXAMPLE 4

The process of Example 1 was repeated except for the use of an etchingsolution comprising 80% of N,N-dimethylformamide, 4% of sodium hydroxideand 16% of methanol heated at 50° C., and conducting etching bycontacting with the cured epoxy adhesive film for 15 minutes. As aresult, a 4-layered multilayer printed circuit board having the sameproperties as in Example 1 was obtained.

EXAMPLE 5

The process of Example 1 was repeated except for the use of an etchingsolution comprising 80% of N,N-dimethylformamide, 0.5% of lithiumhydroxide and 19.5% of methanol heated at 60° C., and conducting etchingby contacting with the cured epoxy adhesive film for 25 minutes. As aresult, a 4-layered multilayer printed circuit board having the sameproperties as in Example 1 was obtained.

COMPARATIVE EXAMPLE 1

The process of Example 1 was repeated except for the use of an etchingsolution containing N,N-dimethylacetamide heated at 50° C. As a result,the cured epoxy adhesive film was not etched and failed to obtain theholes for IVH.

COMPARATIVE EXAMPLE 2

The process of Example 1 was repeated except for the use of an etchingsolution containing methanol heated at 50° C. As a result, the curedepoxy adhesive film was not etched and failed to obtain the holes forIVH.

COMPARATIVE EXAMPLE 3

The process of Example 1 was repeated except for the use as an etchingsolution of an aqueous solution containing 5% of sodium hydroxide and 5%of potassium permanganate heated at 70° C. As a result, the cured epoxyadhesive film was roughened on the surface and failed to expose theinterlayer circuits and also failed to form holes for IVH.

As mentioned above, according to the present invention, since theformation of IVH can be attained simultaneously by chemical etching andsmall holes of 100 μm in diameter or less can be processed, productivityis greatly improved compared with the drill processing of prior arttechnique and processing of very fine diameters becomes possible incontrast to drilling, which is difficult to conduct. Further, since athermosetting copper-clad adhesive resin sheet is used, the pressingstep can be simplified to improve productivity compared with the priorart technique using prepregs. The resin composition used for thethermosetting copper-clad adhesive resin sheet has the same generalproperties as FR-4 which is used in multilayer printed circuit boards.Therefore, the present invention is remarkably useful in the productionof multilayer printed circuit boards mounting various electronic devicesin high density.

Furthermore, the present invention is excellent in safety during theprocess.

What is claimed is:
 1. A process for producing a multilayer printedcircuit board, which comprises the steps of:laminating on an interlayerplate having conductor circuits thereon a thermosetting copper-cladadhesive resin sheet having a copper foil on one side, so as to make theresin side of the sheet contact with the interlayer plate, followed bypressing with heating to cure the resin and to give an integratedlaminate, said thermosetting copper-clad adhesive resin sheet havingbeen prepared by integrating a copper foil with a thermosetting epoxyresin composition comprising (a) an epoxy polymer having aweight-average molecular weight of 100,000 or more, obtained bypolymerizing a bifunctional epoxy resin and a bifunctional halogenatedphenol in an equivalent weight ratio of epoxy group:phenolic hydroxylgroup of 1:0.9 to 1:1.1, (b) a crosslinking agent, and (c) apolyfunctional epoxy resin, and curing the resin to become a B-stagecondition, forming an etching resist on the copper foil of thethermosetting copper-clad adhesive resin sheet, followed by formation offine holes in the copper foil surface by selective etching, removing theetching resist, removing the cured resin layer under the fine holes byetching using an etching solution comprising (A) an amide as a solvent,(B) an alkali metal compound, and (C) an alcohol as a solvent to formvia holes and expose parts of the conductor circuits, plating a metallayer or coating an electroconductive paste so as to electricallyconnecting the conductor circuits of the interlayer plate and an outerlayer copper foil, forming an etching resist on the outer layer copperfoil, followed by formation of wiring circuits on the copper foil byselective etching, and removing the etching resist.
 2. The processaccording to claim 1, wherein the amide as a solvent in the etchingsolution is at least one member selected from the group consisting ofN,N-dimethylacetamide, N,N-dimethylformamide, andN-methyl-2-pyrrolidone, and used in an amount of 50 to 95% by weightbased on the weight of the etching solution.
 3. The process according toclaim 1, wherein the alkali metal compound in the etching solution is atleast one member selected from the group consisting of lithiumhydroxide, sodium hydroxide and potassium hydroxide, and used in anamount of 0.5 to 15% by weight based on the weight of the etchingsolution.
 4. The process according to claim 1, wherein the alcohol as asolvent in the etching solution is at least one member selected from thegroup consisting of methanol, ethanol, diethylene glycol monomethylether, diethylene glycol monobutyl ether, diethylene glycol monoethylether, ethylene glycol monomethyl ether, ethylene glycol monobutyl etherand ethylene glycol monoethyl ether, and used in an amount of 4.5 to 35%by weight based on the weight of the etching solution.
 5. The processaccording to claim 1, wherein the interlayer plate has conductorcircuits on both sides and a pair of the thermosetting copper-cladadhesive resin sheets are laminated on the both sides.